Method of electroplating lead dioxide



United States Patent METHOD OF ELECTROPLATING LEAD DIOXIDE John C.Grigger, Springfield Township, Montgomery County, Pa., assignor toPennsalt Chemicals Corporation, a corporation of Pennsylvania NoDrawing. Filed Dec. 14, 1955, Ser. No. 552,969

20 Claims. (Cl. 204-57) The present invention relates to a novel methodfor electroplating lead dioxide and to a novel bath therefor; and, moreparticularly, the invention relates to a method for electroplating leaddioxide anodically from aslead nitrate bath whereby a markedly improveddeposit of lead dioxide is obtained.

The deposition of lead dioxide anodically is known. Anodicelectroplating of lead dioxide has been accomplished using threediiferent systems: (1) an alkaline bath containing lead tartrate; (2) anacid bath containing lead perchlorate; and (3) an acid bath containinglead nitrate. The use of lead nitrate baths is generally preferredbecause of ease of preparation, safety in use, low cost and uniformityin plating results. The lead nitrate electroplating bath comprises asolutio'n of lead nitrate in water to which may be added copper nitrateto prevent deposition of lead at the cathode. In order to reducegraininess of the lead dioxide deposit, it has been suggested to includein the plating bath such materials as gelatin and glue. The inclusion ofthese materials does reduce graininessthoWever, relatively massive leaddioxide deposits still have a very low fracturing strength, and, whenbroken, reveal many fine cracks radiating from the center to the outsidesurface. In addition, under certain conditions, when plating leaddioxide from baths containing gelatin, anodic gassing occurs resultingin the formation of small pockets and a general irregularity in the leaddioxide surface.

It is the principal object of the present invention to provide a methodfor anodically electroplating lead dioxide which results in markedlyimproved deposits of lead dioxide.

Another object of the present invention is to provide a novel method foranodically electroplating lead dioxide from a lead nitrate bath wherebythe above-mentioned limitations of prior procedures based on the leadnitrate bath are eliminated and whereby a hard, dense, finegrained leaddioxide deposit is obtained.

A further object of the present invention is to provide a novel leadnitrate-containing bath for anodically electroplating lead dioxideproviding improved, hard, dense, finegrained lead dioxide deposits.

Other objects will become apparent from a consideration of the followingspecification and the claims.

The present invention comprises, in the anodic electroplating of leaddioxide from an aqueous acid lead nitrate bath, the improvement whichcomprises carrying outthe electrodeposition in an aqueous acid leadnitrate bath containing a non-ionic surface active agent solubletherein.

It has been found that the inclusion of a non-ionic surfacea'ctive'agent, particularly an alkyl aryl polyether alcohol, in an acidlead nitrate bath for electroplating anodically lead dioxide, markedlyimproves the character of the lead dioxide deposit. The resultingdeposits are free from cracks, are hard and dense and possess a fine,

grained structure, as compared to the deposits resulting from bathscontaining no non-ionic surface active agent or containing other agents,such as gelatin and glue. ..'In

2,945,790 Patented July 19, 1960 addition, the inclusion of thenon-ionic surface active agent readily permits the deposition of leaddioxide having the above-mentioned improved properties in massivelayers, that is layers of at least ,4 thick and upwards of 1 inch thickor more.

The bath employed in accordance with the present invention will comprisean acid, aqueous solution of lead nitrate. In preparing the bath, leadnitrate itself may be added, or the lead nitrate may be formed in situin the bath, as by mixing lead oxide (PbO) with aqueous nitric acid. Inconnection with this latter embodiment, slow addition of the lead oxidein finely-divided fo'rm, to the aqueous nitric acid, with stirring, isadvantageous.

The concentration of lead nitratein the bath, at least at the start ofthe deposition operation, may vary widely. In this connection, theconcentration of the lead nitrate in the bath may range from as low asabout 50 grams per liter to as high as the maximum solubility thereof inthebath at operating temperature, which may be as high as about 700grams per liter. In general, it is desirable, in order to produce a leaddioxide deposit of optimum homogeneity, strength and surfacecharacteristics, to maintain high concentrations of lead nitrate in thevicinity of the anode. Accordingly concentrations above the lower end ofthe range, such as at least about 250 grams per liter and especially atleast about 300 grams per liter, are preferred. In order to facilitatehandling of the bath during shutdown periods and during continuousreplenishing procedures discussed more in detail hereinafter, when thebath may be at or near room temperature, it is also preferable that theconcentration of the lead nitrate not be substantially in excess of itssolubility at these lower temperatures, which may run as high as about400 grams per liter. In the preferred practice of the invention, theconcentration of the lead nitrate will not exceed about 350 grams perliter.

The bath will be maintained on the acid side of neutrality. In general,the more acid the bath the better the operation thereof, and pHs as lowas about 0.8 maybe employed. At acidities appreciably greater than thisacid fuming from the bath becomes excessive, especially at elevatedoperating temperatures. The exact acidity of any particular bathemployed may depend upon the nature of the anode material onto which thelead dioxide is deposited. For example, with base materials that are notrapidly attacked at low pHs, such as tantalum, nickel, and the like, thepH of the bath may range between about 0.8 and about 2.8. However, withmaterials that are more readily attacked by acid, such as steel, it ispreferred to maintain the pH at above about 2 to prevent excessivecorrosion thereof. At pHs above about 3.5

there is a tendency for the formation of an electrical in-,

found that, in the present bath, amounts as low as about 0.5 gram perliter provide significant results, and that amounts between about 0.75and about 2 are optimum.

In accordance with the present invention, there is included in the batha non-ionic surface active agent. Since the bath is highly acid, theagent must be stable and' soluble therein. In this connection, the alkylaryl polyether alcohols, especially the alkyl phenoxy polyoxyethyleneethanols, have been. found to. be particularly grams per liter suitable.The solubility of such materials in the present bath has been found tobe dependent upon the number of ethylene oxide units in thepolyoxyethylene alcohol chain. In other words, the solubility increasesand the cloud point rises as the length of the chain increases untilcompounds having at least about 30 ethylene oxide units in the chain arecompletely soluble even up to the boiling point of water. It has alsobeen found that the solubility of the compounds having shorter chains,that is less ethylene oxide units, can be increased and their cloudpoint raised by the inclusion in the bath of an anionic surface-activeagent, such as a sodium aryl sulfonate, like sodiumtetrahydronaphthalene sulfonate. Hence, by such expedient, alkyl phenoxypolyoxyethylene ethanols having as low as about 4 ethylene oxide unitsmay be used. The inclusion in the bath of an anionic surface-activeagent also permits the attainment of higher cloud points with thosenon-ionic surface-active agents which may be soluble at room temperature(25 C.), but which cloud and precipitate from solution when the bath isheated to a higher temperature. The use of such a mixture ofsurface-active agents is not as efiective in producing a fine-grainedlead dioxide deposit with maximum strength as is the use of the nonionicagent alone. Since the amount of anionic agent required depends upon thesolubility of the non-ionic agent, which solubility in turn depends uponthe number of ethylene oxide units in the polyoxyethylene ethanol chain,it will be seen that the greater the number of ethylene oxide units, upto a point, in the chain the more desirable the non-ionic agent. Thepreferred alkyl phenoxy polyoxyethylene ethanols are those containing atleast about 24 ethylene oxide units in the chain. Compounds containingas many as 40 ethylene oxide units in the chain may be employed.

The amount of non-ionic surface active agent employed to realize theimproved results referred to above, may be .relatively small. Theaddition of as little as about 0.5 gram per liter will providesignificant improvement, and the amount employed may range above thisminimum to as high as about grams per liter or even higher. However,between about 0.75 and about 2 grams per liter represents the optimumconcentration, and little advantage is to be gained by employing largeramounts.

The amount of anionic-surface active agent employed will of coursedepend upon the particular non-ionic agent used and its solubilitycharacteristics. No difliculty will be encountered, however, indetermining the amount of anionic agent required in any particularsituation since it is simply a case of adding the amount whichsolubilizes the non-ionic agent. As a general rule a Weight ratio ofanionic agent to non-ionic agent between about .25 and about 2 to -1,depending upon the solubility of the non-ionic agent, will suflice.

The preparation of the bath itself will present no problem to thosefamiliar with the art, it being only necessary, in accordance with thebroader aspects, to mix the main components in the desired proportionsto provide an aqueous solution thereof. One procedure that has beenfound to be particularly suitable comprises adding finelydivided leadoxide (PbO) slowly to diluted nitric acid, with stirring, after whichthe non-ionic surface-active agent, and any copper nitrate employed, isadded. If the initial lead nitrate solution is not at the concentrationdesired for operation, it may be diluted to the desired level before thenon-ionic surface-active agent and copper nitrate are added. To insuresolution of the components, the mixture may be heated with stirring. Itis desirable to filter the resulting solution, as through sinteredglass, to remove insoluble materials.

The temperature of the bath during operation may vary from roomtemperature to the boiling point. However, deposition is favored atelevated temperatures, and temperatures between about 60 and about 80 C.have been found to be particularly suitable.

During operation of the bath, an anode is immersed in the bath andconnected to a suitable source of current. The anode is the materialonto which the lead dioxide is to be deposited and may be selected froma wide range of conducting materials, including metals like steel,tantalum and nickel, and non-metals such as silicon. The specificpreparation of an anode by electrodepositing lead dioxide onto tantalumand the resulting product forms the subject matter of copendingapplication Serial No. 552,970, filed December 14, 1955 (now abandoned).The shape or form of the anode is immaterial according to the broaderaspects of the invention, and the anode may be a sheet, cylinder, rod,or the like. In copending application Serial No. 552,968, filed December14, 1955 (now Patent No. 2,872,405) are disclosed and claimed thedeposition of lead dioxide on fine wires or screens or other foraminousmetal bodies. The anode employed in the present process may also be insuch form.

A cathode is also provided, and the cathode material may be selectedfrom a wide variety of conducting materials, including lead itself,carbon, and the like. Preferably the cathode is a material non-reactivewith the bath during shutdown, such as carbon.

Upon completion of the circuit lead dioxide begins to deposit at theanode, and the flow of current may be continued until the desireddeposit has been built up. Anode curren density may vary, with densitiesas low as about 5 and as high as about amperes per square foot havingbeen found suitable. Normally, the anode current density will be betweenabout 10 and about 30 amperes per square foot. Cathode current densityis not critical, althought in preferred practice it is generallymaintained higher than the anode current density by a factor of betweenabout 2 and about 10 to :1.

Operation of the bath, of course, removes lead therefrom. Before theconcentration of lead in the bath falls to the point where the bath nolonger operates efficiently it will be necessary, for furtherdeposition, to replenish the bath. The bath may be replenishedperiodically or continuously. In replenishing the bath more lead oxide(PbO) will be added. Some nitric acid is also lost from the bath duringthe operation so that it may be necessary to add further nitric acid asthe bath is replenished in order to maintain the desired pH.

The bath may be periodically or continuously replenished by being flowedthrough another vessel containing a filter bed of, for example, glasswool. Finely-divided lead oxide (PbO) is added to the plating solutionabove the filter bed. The depleted plating solution gradually dissolvesthe lead oxide, passes through the filter bed and is then removed fromthe bottom of this vessel and back to the plating vessel. Nitric acid,when necessary, is also added to the replenishing vessel to maintain thebath at the desired pH.

The present invention will be more readily understood from aconsideration of the following specific example which is given for thepurpose of illustration only and is not intended to limit the scope ofthe invention in any way.

Example 269 m1. of 69.9% nitric acid (266.5 g. HNO are added to 1000 ml.of distilled water. 472 g. of finelydivided lead oxide (PbO) are addedslowly to the diluted nitric acid with stirring. After the addition ofthe lead oxide, the resulting solution is diluted to 2 liters. One andone half grams of copper nitrate, Cu(NO .3H O, and 1.5 g. of nonylphenoxy polyoxyethylene ethanol containing an average of 30 ethyleneoxide units per molecule (Igepal CO-880) are added thereto. Theresulting solution is heated to 75 C. with stirring. The solution isthen allowed to cool and is filtered through sintered glass.

A nickel screen of 16 mesh having 0.012" wire and I? a.) havingdimensions of 3 x 20 is immersed in the bath to a depth of 15". Graphitecathodes are also immersed in the bath. The bath is maintained at 70 C.The nickel screen anode and the graphite cathodes are connected to asuitable source of current and the circuit is completed. A lead dioxideplating thick and 3 /2 x 15% in lateral dimensions and amounting to 6900grams, by weight, is formed on the nickel screen. The deposit is hard,dense and finely-grained.

Considerable modification is possible in the selection of the variousconstituents making up the bath and in the amounts thereof as well as inthe exact techniques followed in carrying out the method withoutdeparting from the scope of the invention.

I claim:

1. In the anodic electrodeposition of lead dioxide from an acid aqueoussolution of lead nitrate the improvement comprising conducting theelectrodeposition with at least about 0.5 gram per liter of analkylaryloxy polyoxyethylene ethanol having from 4 to 40 ethylene oxideunits dissolved in said solution.

2. The method of claim 1 wherein said alkylaryloxy polyoxyethyleneethanol is an alkyl phenoxy polyoxyethylene ethanol.

3. The method of claim 2 wherein said alkyl phenoxy polyoxyethyleneethanol contains at least about 24 ethylene oxide units.

4. In the anodic electrodeposition of led dioxide from an acid aqueoussolution of lead nitrate the improvement comprising conducting theelectrodeposition with at least about 0.5 gram per liter of analkylaryloxy polyoxyethylene ethanol having from 4 to 40 ethylene oxideunits dissolved in said solution and with said solution at a pH betweenabout 0.8 and about 3.5.

5. The method of claim 4 wherein said alkylaryloxy polyoxyethyleneethanol is an alkyl phenoxy polyoxyethylene ethanol.

6. The method of claim 5 wherein said alkyl phenoxy polyoxyethyleneethanol contains at least about 24 ethylene oxide units.

7. In the anodic electrodeposition of lead dioxide from an acid aqueoussolution of lead nitrate the improvement comprising conducting theelectrodeposition at a temperature above room temperature and below theboiling point of the solution and with at least about 0.5 gram per literof an alkylaryloxy polyoxyethylene ethanol having from 4 to 40 ethyleneoxide units dissolved in said solution.

8. The method of claim 7 wherein the temperature is between about 60 andabout 80 C., and wherein the alkylaryloxy polyoxyethylene ethanol is analkyl phenoxy polyoxyethylene ethanol.

9. The method of claim 8 wherein said alkyl phenoxy polyoxyethyleneethanol contains at least 24 carbon atoms.

10. In the anodic electrodeposition of lead dioxide from an acid aqueoussolution of lead nitrate the improvement comprising conducting theelectrodeposition at a temperature above room temperature and below theboiling point of the solution, with at least about 0.5 gram per liter ofan alkyl-aryloxy polyoxyethylene ethanol having from 4 to 40 ethyleneoxide units dissolved in said solution and with said solution at a pHbetween about 0.8 and about 3.5.

' 11. The method of claim 10 wherein the temperature is between about 60and about C., and wherein said alkylaryloxy polyoxyethylene ethanol isan alkyl phenoxy polyethylene ethanol containing at least 24 ethyleneoxide units.

12. In the anodic electrodeposition of lead dioxide from an acid aqueoussolution of lead nitrate the improvement comprising conducting theelectrodeposition with at least about 0.5 gram per liter of analkylaryloxy polyoxyethylene ethanol having from 4 to 40 ethylene oxideunits dissolved in said solution through the agency of an anionicsurface-active agent.

13. The method of claim 12 wherein the alkylaryloxy polyoxyethyleneethanol is an alkyl phenoxy polyoxyethylene ethanol containing betweenabout 4 and about 30 ethylene oxide units and wherein the anionicsurfaceactive agent is a sodium aryl sulfonate.

14. The method of claim 1 wherein the alkylaryloxy polyoxyethyleneethanol is present in an amount between about 0.75 and about 2 grams perliter.

15. A bath for the anodic electrodeposition of lead dioxide comprisingan acid aqueous solution of lead nitrate containing at least about 0.5gram per liter of an alkylaryloxy polyoxyethylene ethanol having from 4to 40 ethylene oxide units dissolved therein.

16. The bath of claim 15 wherein the alkylaryloxy polyoxyethyleneethanol is an alkyl phenoxy polyoxyethylene ethanol, and wherein the pHof the bath is between about 0.8 and about 3.5.

17. The bath of claim 15 wherein the alkylaryloxy polyoxyethyleneethanol is dissolved therein through the agency of an anionicsurface-active agent.

18. The method of claim 1 wherein said alkylaryloxy polyoxyethyleneethanol is nonyl phenoxy polyoxyethylene ethanol.

19. The method of claim 13 wherein said alkyl phenoxy polyoxyethyleneethanol is nonyl phenoxy polyoxyethylene ethanol.

20. The bath of claim 15 wherein the alkylaryloxy polyoxyethyleneethanol is nonyl phenoxy polyoxyethylene ethanol.

References Cited in the file of this patent UNITED STATES PATENTS900,502 Ferchland et al. Oct. 6, 1908 2,492,206 White et al. Dec. 27,1949 2,550,388 Simon et al. Apr. 24, 1951 2,846,378 Hoifmann Aug. 5,1958 FOREIGN PATENTS 456,082 Great Britain Nov. 3, 1936 OTHER REFERENCESIsgarischew et al.: Zeitschrift fiir Elktrochemief vol. 37 (1931), pages359 thru 362.

1. IN THE ANODIC ELECTRODEPOSITION OF LEAD DIOXIDE FROM AN ACID AQUEOUSSOLUTION OF LEAD NITRATE AND IMPROVEMENT COMPRISING CONDUCTING THEELECTRODEPOSITION WITH AT LEAST ABOUT 0.5 GRAM PER LITER OF ANALKYLARLOXY POLYOXYETHYLENE ETHANOL HAVING FROM 4 TO 40 ETHYLENE OXIDEUNITS DISSOLVED IN SAID SOLUTION.